For a well-known vehicle frame structure of a motorcycle, the Japanese Patent No. 2688916 discloses a vehicle frame structure which includes a pair of left and right main frame pieces each having inner and outer plates prepared from a sheet metal and also includes rear arm brackets (swing arm brackets) connected to and extending downwardly from a corresponding rear end of the main frame pieces and having left and right bracket bodies prepared from a cast metal, the left and right bracket bodies being connected with each other by means of cross tubes.
However, since in this known vehicle frame structure the inner and outer plates forming the pair of the left and right main frame pieces are each prepared from a sheet metal, it is difficult to secure dimensional accuracy. Because of this, where an air cleaner is desired to be disposed inside the main frame pieces forming a forked main frame, it is necessary to set a clearance for the mounting of the air cleaner and other components with a dimensional error taken into consideration. As a result, the main frame tends to become bulky in size, resulting in a weight increase.
Also, as shown in FIG. 4c, when an outer plate 61 and an inner plate 62 forming the main frame and prepared from a sheet metal are welded together, the outer and inner plates are overlapped one above the other and then connected together by a fillet welding. Accordingly, in view of the facts that the shape of a welding groove is limited and that it is difficult to position the outer and inner plates 61 and 62, both of which are prepared from a sheet metal and are therefore difficult to be prepared with dimensional accuracy, so as to be overlapped one above the other with no gap formed between the outer and inner plates 61 and 62, the welding workability tends to be lowered. Also, since respective portions in contact with welding joints between the outer and inner plates 61 and 62, which are encompassed by the phantom circles 63 in FIG. 4c, tends to be melted during the fillet welding, a welding torch must be set and held relative to the plate surface at an angle effective to accomplish an optimum welding and, therefore, not only does the welding require time-consuming and cumbersome procedures, but it also requires the skill. In addition, since the welding joint has a step at which the cross-sectional area varies greatly, stress concentration tends to set up easily.
Since the main frame is prepared by welding the outer frame 61 and the inner frame 62, each prepared from the sheet metal, finished products are apt to have large dimensional variations. Accordingly, when the air cleaner is to be disposed in a space inside the main frame, a joint of an air guide port inside the main frame and a joint of the air cleaner will not be smoothly positioned relative to each other, resulting in complicated and time-consuming assemblage.
Yet, when the strength of the main frame prepared from the sheet metal is desired to be locally reinforced, the main frame in its entirety has to be prepared to have an increased thickness and/or a separate element such as a gusset plate must be welded to the area required to be reinforced, resulting in increase of the weight and the manufacturing cost.